The present invention relates to the use of a silicon containing compound as processing aid in the compounding of polyolefin compositions and to a process for compounding a polyolefin composition comprising a silicon containing compound as processing aid.
Polyolefin compositions often comprise several polymeric components, as for example polyolefin resins with different properties, such as different molecular weights, or different content of comonomer. Furthermore, organic and/or inorganic additives such as stabilizers are usually present in a polyolefin composition. The nature and amount of these polyolefin resins and these additives is dependent on the particular use a polyolefin composition is designed for.
In order to obtain a homogeneous end-product, it is important that a very good mixing of the components of the polyolefin composition is obtained, including a very good dispersion of the additives in the polymer matrix. Furthermore, also a high throughput in the processing step, e.g. a high output of an extruder in which the composition is extruded, is desired.
When compounding the polyolefin composition e.g. by extrusion, it is known that mixing, and hence dispersion of the additives, can be improved by higher energy input into the extruder. Of course, also a higher output of the extruder requires higher energy input into the extruder. However, due to an increase of temperature of the polyolefin composition created by friction the amount of energy input and hence the output of the extruder is limited, because usually the polyolefin composition starts degrading at a temperature of about 200° C. It would thus be desirable to have a polyolefin composition allowing for higher energy input into an extruder and hence higher output of the extruder, at lower temperatures thus avoiding degradation of the polyolefin.
Furthermore, in order to avoid degradation of the polyolefin as far as possible and to maximise extruder output, it is also desirable that the composition remains in the extruder as short as possible, i.e. it is desirably to decrease the retention time in the extruder.
Finally, for many applications, such as semiconductive layers in medium or high voltage cables, a high surface quality, including surface smoothness, of the extruded polyolefin composition is required.
It is hence an object of the present invention to provide a processing aid for use in polyolefin compositions, thereby improving the processing properties of the polyolefin compositions, in particular resulting in lower extrusion temperature and higher throughput upon extrusion, decreased retention time in the extruder, and improved surface quality after extrusion.